The subject matter of this invention relates to the field of energy storage and utilization with an improved battery and to an electrode for use therein. More particularly, the invention relates to a battery having a mechanically self-supporting ribbon electrode using disordered active material which may be formed by rapid solidification. The battery electrode stores electrochemical energy for subsequent release to produce an electrical current while maintaining structural integrity during such a cycle.
Some research has been conducted involving hydrogen storage secondary batteries. However, a basic understanding resulting in a viable approach to optimizing such batteries has not been forthcoming in the scientific or patent literature. Examples of such efforts are U.S. Pat. Nos. 3,669,745 and 3,824,131 and a technical paper entitled "A New Type of Reversible Negative Electrode for Alkaline Storage Batteries Based on Metal Alloy Hydrides," 1974, 8th International Power Sources Conference. These research efforts have not resulted in widespread commercial utilization of this battery technology. In fact, the prior research suggests no significant improvement over conventional battery systems such as nickel cadmium. As a result, the hydrogen storage battery system has apparently been ignored or abandoned.
Secondary batteries using a hydrogen rechargeable electrode operate in a different manner than lead acid, nickel cadmium or other battery systems. The hydrogen storage battery utilizes an anode which is capable of reversibly electrochemically storing hydrogen and usually employs a cathode of nickel hydroxide material. The anode and cathode are spaced apart in an alkaline electrolyte. Upon application of an electrical current to the anode, the anode material (M) is charged by the absorption of hydrogen: EQU M+H.sub.2 O+e.sup.- .fwdarw.M--H+OH.sup.-
Upon discharge the stored hydrogen is released to provide an electric current: EQU M--H+OH.sup.- .fwdarw.M+H.sub.2 O+e.sup.-
The reactions are reversible and this is also true of the reactions which take place at the cathode. As an example, the reactions at a conventional nickel hydroxide cathode as utilized in a hydrogen rechargeable secondary battery are as follows:
Charging Ni(OH).sub.2 +OH.sup.- .fwdarw.NiOOH+H.sub.2 O+e.sup.- PA0 Discharging: NiOOH+H.sub.2 O+e.sup.- .fwdarw.Ni(OH).sub.2 +OH.sup.-
The battery utilizing an electrochemically hydrogen rechargeable anode offers important potential advantages over conventional secondary batteries. Hydrogen rechargeable anodes should offer significantly higher specific charge capacities than lead anodes or cadmium anodes. Furthermore, lead acid batteries and nickel-cadmium type secondary batteries are relatively inefficient, because of their low storage capacity and cycle life. A higher energy density should be possible with hydrogen storage batteries than these conventional systems, making them particularly suitable for battery powered vehicles and other mobile applications. Hydrogen storage batteries have not lived up to their potential, however, because of the materials and mechanical structures used.
The materials used for the hydrogen rechargeable anode of the battery are of utmost importance since the anode must efficiently perform a number of functions within useful operating parameters in order to have an efficient charge/discharge cycle. The material must be capable of efficiently storing hydrogen during charging with insignificant selfdischarge until a discharge operation is initiated. Since reversibility of the charge/discharge reactions is necessary, a highly stable bonding of hydrogen to the storage sites of the anode is not desired. On the other hand, it is also undesirable if the bonds between the hydrogen atoms and the anode material are too unstable. If the bonds are too unstable during charging, the dissociated hydrogen atoms may not be stored by the anode, but may recombine to form hydrogen gas such as in the electrolysis of water. This can result in low efficiencies, loss of electrolyte and inefficient charging.
Another important characteristic of the hydrogen rechargeable electrode is its structure. Prior art hydrogen storage materials use a binder material to physically hold the hydrogen storage material together and in electrical contact with a grid collector. Since the binder material is not itself an active hydrogen storage material, its use decreases the capacity of the cell. The increase in weight of inactive material also decreases the energy density of a cell using the electrode.
As an electrode charges and discharges, the hydrogen storage material expands and contracts. These volumetric changes can cause cracking and disintegration of the electrode's structural integrity. The electrode tends to fail if the hydrogen storage material loses electrical contact with, or falls away from, the collector grid. Using more binder material will delay this problem and extend cycle life of the cell, but the capacity of the cell decreases.
Another problem experienced by some hydrogen rechargeable electrodes is low charge and discharge rates. Increasing the surface area of an electrode shortens the hydrogen diffusion lengths and increases the charge and discharge rates. For prior art electrodes, increasing the surface area is achieved by increasing the porosity. Increasing the porosity, however, allows the electrolyte to corrode and degrade the electrode's structural integrity. Again, this leads to early electrode failure as the material disintegrates and falls away from the grid collector. Attempts to alleviate this problem by coating the surface to prevent the active material from falling away is not helpful since any coating would decrease the needed porosity.
Many previous attempts to utilize hydrogen in secondary batteries have proven to be unsuccessful because of the limiting factors of structural and electrical integrity in using hydrogen storage materials. The invention herein provides a new and improved battery having an electrode with an active material and mechanical structure allowing for high charge and discharge rates, efficient reversibility, high electrical efficiency, bulk hydrogen storage without substantial structural change or poisoning, mechanical integrity over long cycle life, and deep discharge capability.